An important component of disposable absorbent articles such as diapers is an absorbent core structure comprising water-absorbing polymers, typically hydrogel-forming water-absorbing polymers, also referred to as absorbent gelling material, AGM, or super-absorbent polymers, or SAP's. This polymer material ensures that large amounts of bodily fluids, e.g. urine, can be absorbed by the article during its use and locked away, thus providing low rewet and good skin dryness.
Especially useful water-absorbing polymers or SAP's are often made by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g., sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like in the presence of relatively small amounts of di- or poly-functional monomers such as N,N′-methylenebisacrylamide, trimethylolpropane triacrylate, ethylene glycol di(meth)acrylate, or triallylamine. The di- or poly-functional monomer materials serve to lightly cross-link the polymer chains thereby rendering them water-insoluble, yet water-absorbing. These lightly crosslinked absorbent polymers contain a multiplicity of carboxylate groups attached to the polymer backbone. It is generally believed that the neutralized carboxylate groups generate an osmotic driving force for the absorption of body fluids by the crosslinked polymer network. In addition, the polymer particles are often treated as to form a surface cross-linked layer on the outer surface in order to improve their properties in particular for application in baby diapers, adult incontinence articles and fem-care articles.
Water-absorbing (hydrogel-forming) polymers useful as absorbents in absorbent members and articles such as disposable diapers need to have adequately high absorption capacity, as well as adequately high gel strength. Absorption capacity needs to be sufficiently high to enable the absorbent polymer to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. Together with other properties of the gel, gel strength relates to the tendency of the swollen polymer particles to resist deformation under an applied stress. The gel strength needs to be high enough in the absorbent member or article so that the particles do not deform and fill the capillary void spaces to an unacceptable degree causing so-called gel blocking. This gel-blocking inhibits the rate of fluid uptake or the fluid distribution, i.e. once gel-blocking occurs, it can substantially impede the distribution of fluids to relatively dry zones or regions in the absorbent article and leakage from the absorbent article can take place well before the water-absorbing polymer particles are fully saturated or before the fluid can diffuse or wick past the “gel blocking” particles into the rest of the absorbent article. Thus, it is important that the water-absorbing polymers (when incorporated in an absorbent structure or article) maintain a high wet-porosity and have a high resistance against deformation thus yielding high permeability for fluid transport through the swollen gel bed. On the other side it is also beneficial that the swollen gel bed has narrow pores in order to allow efficient fluid distribution by wicking mechanisms.
Absorbent polymers with relatively high permeability can be made by increasing the level of internal crosslinking or surface crosslinking, which increases the resistance of the swollen gel against deformation by an external pressure such as the pressure caused by the wearer, but this typically also reduces the absorbent capacity of the gel which is undesirable. It is a significant draw back of this conventional approach that the absorbent capacity has to be sacrificed in order to gain permeability. The lower absorbent capacity must be compensated by a higher dosage of the absorbent polymer in hygiene articles which for example leads to difficulties with the core integrity of a diaper during wear. Hence, special, technically challenging and expensive fixation technologies are required to overcome this issue in addition to the higher costs that are incurred because of the required higher absorbent polymer dosing level.
Because of the trade-off between absorbent capacity and permeability in the conventional approach, it is extremely difficult to produce absorbent polymers that show improved properties regarding absorbent capacity and permeability versus what is described by the following empirical relation:Log(CS-SFC′/150)≦3.36−0.133×CS-CRC  (1)and it is even more difficult to produce absorbent polymers that show improved properties regarding absorbent capacity and permeability versus what is described by the following empirical relation:Log(CS-SFC′/150)≦2.5−0.095×CS-CRC  (2)
It is therefore very desirable to produce absorbent polymers that fulfil the following relations (3) or (4) or preferred (3) and (4):Log(CS-SFC′/150)>3.36−0.133×CS-CRC  (3)Log(CS-SFC′/150)>2.5−0.095×CS-CRC  (4)
In all relations above, CS-SFC′=CS-SFC×107 and the dimension of 150 is [cm3s/g].
If in the relations (1) through (4) above the CS-CRC is replaced with the CCRC as defined herein, all of the relations remain valid. It is therefore particularly desirable to produce absorbent polymers that fulfil the following relations (5) or (6):Log(CS-SFC′/150)>3.36−0.133×CCRC  (5)Log(CS-SFC′/150)>2.5−0.095×CCRC  (6)
In relations (5) and (6) above, CS-SFC′=CS-SFC×107 and the dimension of 150 is [cm3s/g]. Log is the logarithm to the basis 10.
Often the surface cross linked water-absorbing polymer particles are constrained by the surface-cross linked shell and cannot absorb and swell sufficiently, and/or the surface-cross linked shell is not strong enough to withstand the stresses of swelling or the stresses associated with performance under load.
As a result thereof the coatings or shells of the water-absorbing polymers, as used in the art, including surface cross-linking ‘coatings’, break when the polymer swells significantly or it has been in a swollen state for a period of time. Often the coated and/or surface-cross linked water-absorbing polymers or super-absorbent materials known in the art deform significantly in use thus leading to relatively low porosity and permeability of the gel bed in the wet state.
The present invention thus has for its objective to provide absorbent structures with improved water-absorbing material having a more advantageous modification of the surface whose integrity is preserved during the swelling and preferably also during the lifetime of the hygiene article manufactured using this absorbent polymer, and/or such water-absorbing materials obtainable by a specific improved process that provides for these improved properties.
EP-A-0 703 265 teaches the treatment of hydrogel with film-forming polymers such as acrylic/methacrylic acid dispersions to produce abrasion-resistant absorbents. The treating agents identified include polyurethanes. However, the absorbent particles obtained therein give unsatisfactory absorption values, especially with regard to CCRC, CS-CRC and CS-SFC. More particularly, the reference cited does not teach how to produce uniform coatings that retain their mechanical properties to a sufficient degree during swelling and during use.
The older PCT-applications WO 2005/014697, WO 2005/014067, US 2005/031868, US 2005/031872 and US 2005/043474 teach the spray-coating of hydrogel with elastic-film-forming polymers in a fluidized bed reactor. However, there is no teaching about adding an antioxidant. There is no teaching on the optimum annealing time in the heat treatment step and there is no teaching on advantageous coalescing agents.
In general, the handling of water-absorbing polymeric particles at higher temperatures is done under an inert gas or a vacuum is applied to reduce performance losses of the hydrogel. Both results in a high apparative effort. Another possibility is to work at lower temperatures, which results in longer reaction time and a low production output. The objective of the invention accordingly is to provide a process for producing water absorbing polymeric particles with a good space-time yield. It is an objective of the invention to provide a process with a short heat-treatment step. It is a further objective of the invention to provide a method for determination of the optimum heat-treatment time and to provide a process for production of performance optimized water-absorbing polymeric particles.
The objective of this invention accordingly is to provide absorbent structures with water-absorbing polymeric particles having high core shell centrifuge retention capacity (CS-CRC), high core shell absorbency under load (CS-AUL) and high core shell saline flow conductivity (CS-SFC), the water-absorbing polymers having to have high core shell saline flow conductivity (CS-SFC) in particular.
The objective of this invention accordingly is to provide absorbent structures with water-absorbing polymeric particles having high cylinder centrifuge retention capacity (CCRC), high core shell absorbency under load (CS-AUL) and high core shell saline flow conductivity (CS-SFC), the water-absorbing polymers having to have high core shell saline flow conductivity (CS-SFC) in particular.